1. Field of the Invention
The present invention relates generally to engineering of conductive films, and more specifically to methods of engineering of composite thin conductive films by atomic layer deposition technology.
2. Description of the Related Art
Thin conductive films are used in the fabrication of integrated circuits to route signals through and between many of the device elements of an integrated circuit, including interconnect lines, capacitor and gate electrodes, and contacts to the source and drain transistor regions. Traditionally, interconnect lines are fabricated from aluminum and are embedded in SiO2 dielectric insulation. In recent years leading microelectronics manufacturers have begun the implementation of copper (Cu) and low dielectric constant (low-K) materials to enable improvement in device performance by lowering the signal propagation time delay. An important aspect of copper metallization is the need for a diffusion barrier to completely encapsulate the Cu interconnect lines in order to prevent Cu diffusion into the low-k material, which can degrade performance and yield. The barrier material and deposition method need to be carefully designed to avoid compromising the resistivity and reliability of the interconnect system. For example, the effective copper conductance across the interconnect line can be maximized by the use of a thin, uniformly deposited barrier layer. As illustrated in FIG. 1, the barrier thickness requirements rapidly approach less than 75 Å to retain 90% of the conductivity benefit of copper at feature size (FS) of 0.18 μm and below, and the historical industry trend is continuous scaling to smaller feature sizes and higher aspect ratios (ratio of depth to width of the features). Copper interconnect lines are fabricated with a damascene process, where the Cu is deposited inside trench and via openings defined in the insulating material. Advanced metallization technologies deposit the Cu simultaneously in the lines and vias in a dual damascene process, where the aspect ratios may exceed 10:1. Therefore, innovative ultra-thin barrier material is needed with good diffusion barrier properties and low resistivity, that is 100% conformal along aggressively scaled topographies. Other key integration requirements are low resistivity, good adhesion to the both Cu and low-K materials, ability to promote preferred Cu texture, low deposition temperature, and thermal stability within the constraints of the back-end-of-the-line (BEOL) thermal budget, typically less than or ˜650° C., and long-term device operation. These functions put conflicting requirements on the material properties of the barrier, and this may be difficult to accomplish by just one film composition. One approach to meet these requirements is to employ barrier films composed of multiple sub-layers with tuned properties. In this respect, there is a need for composite barrier films. There also is a suitable technique, atomic layer deposition (ALD), which has the ability to grow ultra-thin conformal films with high precision control of thickness and to engineer the composition (T. Suntola, Thin Solid Films 216, p. 84, 1992, Sneh et al., Thin Solid Films, Vol. 402/1-2, pp. 248–261, January 2002).
Titanium nitride (TiN) is the commonly used barrier material with aluminum metallization. However, TiN is not likely to meet the challenges associated with extendibility to sub-0.18 μm and copper, because its diffusion barrier properties deteriorate as the film thickness is scaled down. Barrier materials naturally superior to TiN are the nitrides of the heavier refractory metals tantalum and tungsten.
TaNx-based materials have become the material of choice for copper barrier in present day IC manufacturing. TaN is a good barrier, however it has high resistivity and poor adhesion to Cu. On the other hand, Ta has good adhesion to Cu and lower resistivity, but it fails as a barrier. It has been established that some N incorporation in the metal film is essential for good barrier performance. However, it has been also established that the barrier/dielectric and barrier/Cu adhesion have conflicting dependencies on the percent of N in the barrier (Edelstein et al., IEDM Conference Proceedings 2001, IEEE, p. 9). Thus, some barrier film engineering is necessary and TaNx, or TaNx/Ta bi-layers have emerged as the barriers of choice (Edelstein et al., IEDM Conference Proceedings 2001, IEEE, p. 9).
The dominant technique for deposition of these advanced Cu barriers is ionized physical vapor deposition (IPVD). IPVD is a variant of sputter-deposition technology, where the sputtered atoms are ionized by the application of a magnetic field to improve step coverage. PVD techniques have the advantage of maturity, long history of application in the IC industry, good control of film composition. However, even the state of the art TaNx film deposition technologies only approach 33% step coverage in currently manufactured device geometries. Additionally, PVD deposited composite films containing more than one metal element may suffer from non-uniform film composition along the side wall of the features.
For improving the step coverage the dominant technique for deposition of thin films in the art has been chemical vapor deposition (CVD), which has proven to have superior ability to provide uniform even coatings, and to coat relatively conformally into vias and over other high-aspect ratio and uneven features in wafer topology. As device density has continued to increase and geometry has continued to become more complex, even the superior conformal coating of CVD techniques has been challenged, and new and better techniques are needed. Additionally, CVD processes generally have very narrow window for fabrication of multi-component films and the control over film composition is not as precise as in PVD. Furthermore, issues related to the CVD system complexity, cost and maintenance are encountered.
The approach of a variant of CVD, Atomic Layer Deposition (ALD) has been considered for improvement in uniformity, conformality, and control of film composition, especially for low temperature deposition, appropriate for interconnect BEOL processing requirements.
Atomic Layer Deposition is emerging as a promising candidate to extend the abilities of CVD techniques, and is under rapid development by semiconductor equipment manufacturers to further improve characteristics of chemical vapor deposition. ALD was originally termed Atomic Layer Epitaxy by T. Suntola (Suntola and J. Ashton, U.S. Pat. No. 4,058,430). Generally, ALD is a process wherein conventional CVD processes are divided into single deposition steps, wherein each separate deposition step theoretically goes to saturation at a single or a fraction of a monolayer thickness, and self-terminates.
The deposition is the result of chemical reactions between reactive molecular precursors and the reactive surface of the growing film. In similarity to CVD, elements composing the film are delivered as molecular precursors. The net reaction deposits the pure desired film and eliminates the byproducts. In the case of CVD the molecular precursors are fed simultaneously into the CVD reactor. A substrate is kept at temperature that is optimized to promote chemical reaction between the molecular precursors concurrent with efficient desorption of the byproducts.
For ALD applications, the molecular precursors are introduced into the ALD reactor separately. This is practically done by flowing one precursor at a time, i.e., a metal precursor—MLx(M=Ti, Al, W, Ta, Si etc.) that contains a metal element—M which is bonded to atomic or molecular ligands—L to make a volatile molecule. The metal precursor reaction is typically followed by inert gas purging to eliminate this precursor from the chamber prior to the separate introduction of the other precursor. The second type of precursor is used to grow the desired film and restore the surface reactivity towards the metal precursor.
Most ALD processes have been applied to deposit compound films. In this case the second precursor is composed of a desired (usually nonmetallic) element (i.e., O, N), and hydrogen such as, for example H2O or NH3. Specifically, oxides are formed when H2O is used, and nitrides, when NH3 is employed. FIG. 2 illustrates the process for the formation of TiN and TaNx films by ALD. TiN is grown by alternating TiCl4 and NH3 pulses. Purge step is performed after every pulse to remove the precursor from the deposition chamber. Similarly, TaNx is deposited from sequential TaCl5 and NH3 pulses. Methods for thermal and plasma assisted ALD processes now exist for the deposition of elemental materials (J. W. Klaus et al., Appl. Phys. Lett. 70, 1997, p. 1092; Imai et al., 1993). Plasma assisted ALD can also be used for the deposition of compound films (O. Sneh, U.S. Pat. No. 6,200,893, 2001).
The sequence of surface reactions that restores the surface to the initial state is called the ALD deposition cycle. By repeating the ALD cycles, films can be layered down in equal metered sequences that are all identical in chemical kinetics, deposition per cycle, composition and thickness. Self-saturating surface reactions make ALD insensitive to transport non-uniformity either from flow engineering or surface topography (i.e., deposition into high aspect ratio structures). Non uniform precursor flux can only result in different completion time at different areas of the wafer. However, if each of the reactions is allowed to complete on the entire substrate area, the different completion kinetics has no disadvantage.
Some research has been done on conducting film deposition by ALD for barrier applications (A. Satta et al., Mat. Res. Soc. Symp. Proc. Vol. 612, 2000, p. D6.5.1, MRS; P. Martensson et al., J. Vac. Sci. Technol. B 17, p. 2122, 1999) however composite conductive film engineering has not been documented. A related effort in the integration of copper interconnects using atomic layer deposition has included the concept of a barrier and a Cu pre-seed by ALD followed by a CVD seed (S. Lopatin et al., U.S. Pat. No. 6,368,954, 2002). This approach is related art to the concept disclosed here by commonality of application, but does not address the detailed design and structure of the ALD barriers.
There is a need for diffusion barrier films that simultaneously address and satisfy the requirements for incorporation in advanced microelectronics devices, such as, good diffusion barrier properties, low resistivity, 100% conformallity along aggressively scaled topographies, good adhesion to both the Cu and low-K materials, ability to promote preferred Cu texture, with deposition temperature and thermal stability within the constraints of the BEOL thermal budget, and the like.